Messages in this thread | | | Date | Tue, 18 Mar 2008 00:23:04 +0300 | From | Oleg Nesterov <> | Subject | Re: [PATCH] fix misplaced mb() in rcu_enter/exit_nohz() |
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On 03/17, Paul E. McKenney wrote: > > On Mon, Mar 17, 2008 at 11:17:41PM +0300, Oleg Nesterov wrote: > > (to clarify: my question is completely offtopic to this patch) > > On 03/17, Paul E. McKenney wrote: > > > On Mon, Mar 17, 2008 at 09:30:47PM +0300, Oleg Nesterov wrote: > > > > I'm not sure the code below is up to date, but what I have in > > > > arch/s390/kernel/time.c is: > > > > > > > > stop_hz_timer: > > > > > > > > cpu_set(cpu, nohz_cpu_mask); > > > > > > > > if (rcu_needs_cpu(cpu) || local_softirq_pending()) { > > > > cpu_clear(cpu, nohz_cpu_mask); > > > > return; > > > > } > > > > > > > > Don't we need smp_mb() after cpu_set() ? > > > > > > S390's memory model is quite strong, so it might not be needed. > > > > OK, in that case we shouldn't worry. > > I don't know if I would go -that- far. ;-) > > > > In any > > > case, if needed, it goes -before- the cpu_set(), because the problems > > > would arise if prior RCU read-side critical sections were to be reordered > > > to follow this cpu_set(), right? > > > > No, but it is very possible I missed something. > > > > What if rcu_needs_cpu(cpu) is executed before cpu_set(cpu, nohz_cpu_mask)? > > It can miss rcu_start_batch() -> rcp->cur++ and return false, but at the > > same time rcu_start_batch() may see nohz_cpu_mask without this CPU. > > If you mean that the rcu_needs_cpu() executes before the cpu_set() in > the code fragment above, while the rcu_start_batch() executes on some > other CPU?
Yes, and __rcu_pending() sees the old value of ->cur.
IOW. Suppose that this CPU reads rcp->cur out of order. To simplify, let's suppose that stop_hz_timer() on CPU_0 in fact does
xxx = rcu_needs_cpu(cpu); // false
// ---- WINDOW ------
cpu_set(cpu, nohz_cpu_mask);
if (xxx || local_softirq_pending()) { ... abort ... }
...proceed...
Another CPU does rcu_start_batch() in the window above. In that case rcp->cpumask will include CPU_0, and the grace period can't be completed untill CPU_0 is "woken".
Oleg.
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